[fix](nereids) Prefer DATETIMEV2 over TIMESTAMPTZ when binding *_diff functions on non-literal string args

[lets-order-some-fries]

What problem does this PR solve?

Issue Number: close #66120

Related PR: #64127

Problem Summary:

DATEDIFF and the other eleven *_diff scalar functions silently return off-by-one (or off-by-N) results when (1) an argument is a string-typed non-literal — a varchar column, subquery projection slot, or UNION-ALL output slot — and (2) the session time_zone is not UTC.

All 12 functions list their TIMESTAMPTZ signature first in SIGNATURES. The timezone-coercion penalty in SearchSignature.doMatchTypes only fires for literals it can inspect (ExpressionUtils.getLiteralAfterUnwrapNullable, added in #64127), so for a varchar SlotReference every candidate signature ties and the tie-break keeps the first-listed candidate — TIMESTAMPTZ. The varchar value is then cast through timestamptz(6), which treats it as session-local time and shifts it to UTC, so day-number arithmetic runs on the shifted value.

Fix, as proposed by @Baymine in #66120: move the TimeStampTz signature to last in each of the 12 *Diff files. Literal-with-timezone arguments still route to TIMESTAMPTZ via the timeZoneCoersionScore branch; typed TIMESTAMPTZ / DATETIMEV2 / DATEV2 arguments still bind their identical-match signatures. Only the previously order-decided tie for non-literal string arguments changes: TIMESTAMPTZ → DATETIMEV2 (wall-clock semantics, independent of session time zone).

New unit test DiffFunctionSignatureTest covers all 12 functions in both directions: varchar slots must bind DATETIMEV2 (fails on master before this change) and TIMESTAMPTZ slots must still bind TIMESTAMPTZ. Happy to also add a SQL regression suite mirroring the issue's reproducer (varchar column / subquery slot / UNION-ALL slot under +08:00) if wanted — omitted here because the .out expected-files need a cluster run to generate.

Release note

Fixed DATEDIFF and the other *_diff functions returning wrong results on varchar columns and subquery slots when the session time_zone is not UTC.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes. For *_diff calls whose string-typed argument is NOT a literal, the argument now binds to DATETIMEV2 (wall-clock) instead of TIMESTAMPTZ (UTC-shifting). That re-bind is the fix; literal arguments and typed date/datetime/timestamptz arguments are unaffected.

• Does this need documentation?

  • No.
  • Yes.

[hello-stephen]

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